The invention relates to methods and devices for balancing a three-phase system formed by means of an invertor from a direct-current (dc) voltage source, the output of said invertor being provided with a filter followed by a neutral grounding transformer.
A device for generating a balanced three-phase system with a loadable neutral conductor is known from European patent 0 208 088. That device comprises an invertor arrangement and a voltage-transforming means, and more particularly a floating three-phase voltage transformer with a Y-connection in the secondary circuit, which at the input end is connected to the invertor arrangement. Moreover, a filter is connected in series with the voltage-transforming means. A setpoint generator specifies three angularly symmetrical setpoints. Associated with each setpoint and alternating-current (ac) output of the invertor arrangement is a control system that is followed by a drive unit. The invertor arrangement consists of a pulse-width-modulated three-phase bridge invertor. A decoupling network mirroring the structure of the voltage-transforming means forms transformed instantaneous values, which correspond to a voltage system at the input of the voltage-transforming means, from instantaneous values corresponding to the phase-to-neutral voltages at the output of the series circuit. The control system and the decoupling network are connected together in such a way that they supply control voltages for each pair of branches from the setpoints and from measurands for the phase-to-neutral voltages of the output voltage system. These control voltages act upon an output of the invertor arrangement, and correct the deviation of the transformed measurands from the setpoints. With this device, instantaneous unbalances occurring at the terminals of the output voltage system can be quickly corrected. This can occur because each pair of the invertor arrangement is acted upon by a dedicated drive unit with a control voltage which rapidly changes, as in an unbalanced voltage system. The decoupling network permits conversion of the measurands to equivalent measurands of a system without a neutral conductor. These measurands which are converted are linked to the loadable neutral to be converted in the secondary circuit. As a result, equivalent actual values of a fictitious voltage system are available for control at the input of the voltage-transforming means. The control voltages formed are unbalanced.
From the article "Coordinate Transformations for Multi-term Regulation Systems for the Compensation and Symmetrization of Three-Phase Supplies" (Koordinatentransformationen fur MehrgroBen-Regelsysteme zur Kompensation und Symmetrierung von Drehstromnetzen), published in Siemens Forschungs- und Entwicklungs-Berichte, vol. 6 (1977), No. 1, pp. 29-38, a control system is known which is designed to control the thyristors of a reactive-current compensation system at all times so that the reactive-current loading of the electric power system is as low and as constant as possible and that the load on the power system overall is as nearly balanced as possible. The control system (FIG. 19 of the article) comprises two vector processors which convert the furnace currents and the thyristor currents into active- and reactive-current components of a positive and negative phase-sequence system. The active- and reactive-current components of the positive and negative phase-sequence system of the furnace currents serve as command references for the control of the compensation system. Each of the two vector processors comprises on the input side a transformation device (transformation from a three-phase system into two vectors), each of whose outputs is followed by a vector identifier followed by a vector rotator. The outputs of the vector rotators are linked to a transformation device (transformation of static vectors into positive phase-sequence system and negative phase-sequence system components). The article also gives examples for implementation of a vector identifier and a vector rotator. For the transformations, transformation matrices are given. Since the control system essentially has to process only command-reference changes, and since disturbance variations occur only as slight fluctuations of the bus voltage, provision is made for precontrol so that the controllers need only correct minor deviations. With this control system, the reactive component of the positive phase-sequence system of the line current is held to as small and as constant a value as possible, while the components of the negative phase-sequence system are maintained at the same time to very small values. If, in addition, the system load is to be balanced, the thyristor circuits must be controlled independently in all phases.
The present invention is directed to the problem of developing a method and a device for balancing a three-phase device such that the three-phase system formed can be balanced rapidly and simply with means of a multivariable control system.